Rotors, such as for gas turbine engines and the like, are typically subjected to high stresses and high temperatures. Their design, construction, and the materials from which they are made often dictate operating limits for the apparatus in which they are employed. Extensive efforts have been made over the years to develop new alloys, new fabrication techniques, and new component designs which permit operation of these rotors at higher operating temperatures and/or which lead to lighter weight, longer lived components with all their attendant advantages.
The most common rotor design used today in high temperature, high speed applications, such as in gas turbine engines for jet aircraft, comprises a disk with blades or airfoils mechanically attached to the disk rim. The alloy used for the disk is selected to meet the requirements of high tensile strength and good low cycle fatigue resistance. Such properties are found in, for example, fine equiaxed grain superalloy material. The airfoils, which are exposed to the higher temperature of the gas path and greater centrifugal loads, are stress rupture and creep limited; thus, they are made from materials having good stress rupture and creep characteristics which are typical of coarser grained materials. No alloy processed to a singular microstructure could give optimum properties demanded by the conditions in both the disk section and the airfoils in advanced turbine engines without placing an extreme tax on either the designer's skill or the component weight. One piece, integral centrifugal rotors, such as radial inflow turbine rotors, pose similar problems.
There are many techniques disclosed in the prior art for fabricating integrally bladed rotors using different materials for the blades and the hub or disk. (The phrase "different material", as used in this specification and the claims, refers to materials having different properties but which may or may not have the same element composition. Thus, alloys having the same element composition and which are processed differently so as to exhibit properties different from each other are considered "different materials"). Two such techniques are described in U.S. Pat. Nos. 4,096,615 and 4,270,256. In both of those patents hot isostatic pressing is used to diffusion bond blades of one material to a disk of another material. Both patents recognize the difficulty in maintaining precise dimensional controls between adjacent airfoil components. Both include relatively complex procedures for forming an integral ring of blades. A radially inwardly facing surface of the ring is machined to a precise diameter to form a bonding surface adapted to mate with the radially outwardly facing bonding surface of a rotor disk made from a different material than the blades. The ring is positioned over the disk; and oxygen and other contaminants are removed from the bonding surfaces by vacuum outgasing, followed by sealing external joint lines with braze material. Hot isostatic pressing is then used to diffusion bond the blades to the disk.
Aside from the complexities involved in positioning the blades about the disk prior to hot isostatic pressing, as evidenced by the procedures involved in the above discussed U.S. Pat. Nos. 4,096,615 and 4,270,256 patents, an even more basic problem exists. Prior art diffusion bonding methods do not consistently result in wholly satisfactory solid state diffusion bonds. The bond will sometimes be very good, but under apparently identical conditions sometimes turn out totally unacceptable. Repeatability is missing. The present invention overcomes this significant shortcoming and also provides a relatively simple method for positioning blade elements about the disk in preparation for diffusion bonding.
In addition to the patents discussed above, Lazar et al, U.S. Pat. No. 3,122,823, uses conventional forging techniques to forge an integrally bladed rotor. A more recent development is the Gatorizing.RTM. isothermal forging method useful with high temperature superalloys as described in commonly owned U.S. Pat. No. 3,519,503, the teachings of which may be used in conjunction with commonly owned U.S. Pat. Nos. 4,074,559 and 4,265,105 which describe apparatus which may be used to forge integrally bladed rotors from superalloys. Other patents relevant to the fabrication of dual material rotors are: U.S. Pat. Nos. 2,479,039; 2,703,922; 2,894,318; 3,047,936; 3,598,169; 3,905,723; 4,051,585; 4,063,939; 4,097,276; and 4,175,911 (radial turbine wheel).
The state of the art for integrally bladed superalloy rotors is more fully described in a paper titled "Fabrication and Heat Treatment of a Nickle-Base Superalloy Integrally Bladed Rotor for Small Gas Turbine Applications" by Hughes, Anderson and Athey published on June 22, 1980 in Modern Developments in Powder Metallurgy--Volume 14 Special Materials, Published by Metal Powder Industries Federation. That paper discusses the fabrication of an integrally bladed rotor by the aforementioned Gatorizing process using a single superalloy throughout. Desired differences in properties between the airfoil and hub portion of the rotor are obtained by directionally recrystallizing the airfoils from their tips to a desired distance into the rim of the hub. The hub retains its fine equiaxed grains. This step is followed by further heat treatment cycles.
Better techniques for forming integrally bladed rotors having blades made from a different material than the disk are still needed.